Drug delivery devices for setting and dispensing a single or multiple doses of a liquid medicament are as such well-known in the art. Generally, such devices have substantially a similar purpose as that of an ordinary syringe.
Drug delivery devices, in particular pen-type injectors have to meet a number of user-specific requirements. For instance, with patient's suffering chronic diseases, such like diabetes, the patient may be physically infirm and may also have impaired vision. Suitable drug delivery devices especially intended for home medication therefore need to be robust in construction and should be easy to use. Furthermore, manipulation and general handling of the device and its components should be intelligible and easy understandable. Moreover, a dose setting as well as a dose dispensing procedure must be easy to operate and has to be unambiguous.
Typically, such devices comprise a housing or a particular cartridge holder, adapted to receive a cartridge at least partially filled with the medicament to be dispensed. The device further comprises a drive mechanism, usually having a displaceable piston rod which is adapted to operably engage with a piston of the cartridge. By means of the drive mechanism and its piston rod, the piston of the cartridge is displaceable in a distal or dispensing direction and may therefore expel a predefined amount of the medicament via a piercing assembly, which is to be releasably coupled with a distal end section of the housing of the drug delivery device.
The medicament to be dispensed by the drug delivery device is provided and contained in a multi-dose cartridge. Such cartridges typically comprise a vitreous barrel sealed in distal direction by means of a pierceable seal and being further sealed in proximal direction by the piston. With reusable drug delivery devices an empty cartridge is replaceable by a new one. In contrast to that, drug delivery devices of disposable type are to be entirely discarded when the medicament in the cartridge has been completely dispensed or used-up.
Document WO 2006/076921 A1 describes a handheld mechanical injection device, wherein the medicament is expelled through an injection needle by release of a power reservoir in the device. The power reservoir is fully or partially released by actuation of a user operable release member being positioned at or near an upper end of the injection device. The power reservoir is arranged between the housing of the device and a dose setting member in such a way that when the dose setting member is rotated, energy is accumulated in the power reservoir.
The power reservoir comprises a torsion spring formed as a helical spring extending coaxially with an associated piston rod. The torsion spring is energized by rotating a ratchet which is operatively connected to the housing of the injection device when the dose to be injected is being set. In a dose setting position the ratchet is operatively connected with a housing part via a ratchet arm. Energy accumulated in the torsion spring is released by displacing the ratchet axially whereby it is released from its connection with the housing part in that the ratchet arm is moved into the housing part whereby the piston rod is allowed to rotate thereby expelling a set dose of medicament.
The torsion spring in combination with a ratchet allows to accumulate and to store mechanical energy in the device upon setting of a dose. However, if a set dose is too large a dose correction or dose decrementing operation is required. In order to enable dose correction or dose decrementing the ratchet must either temporally disengage or must be transiently overruled. Moreover, a dose decrementing actuation may act in unison with the biased spring element which may have a negative impact on the general handling of the device and its operability to modify the size of a dose.
Additionally, demands for dosing accuracy may further require that the ratchet successively engages with every standard unit of the medicament to be dispensed. Since typical handheld drug delivery devices are limited in geometrical size the mutually inter-engaging components of such a ratchet have to be rather small and filigree in order to provide a required dosing accuracy. However, such tiny or filigree mechanical components impose a rather complicated assembly process and may be rather susceptible to mechanical loads or to mechanical stress when the device is operated.